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Nature and Extent of Directly Measured Denitrification Losses from Some Irrigated Vegetable Crop Production Units¹

ABSTRACT

Directly measured denitrification N flux from field study sites in irrigated vegetable production units, ranged from 0.024 to 1.92 kg N ha^–1 day^–1 subsequent to irrigation, and values up to 3.60 kg N ha^–1 day^–1 were observed after rain. The N denitrification flux was measured as nitrous oxide (N₂O) evolution from locations within each study site treated with actylene (C₂H₂) to establish C₂H₂ concentrations close to 1% (vol/vol) in the soil pore space. Denitrification N flux showed a pronounced dependence on soil-water suction. Peak fluxes were observed for suctions between 50 and 100 mbars and were associated with field-measured Eh (redox potential) values between 400 and 650 mV. Flux decreased to < 0.096 kg N ha^–1 day^–1 when suction increased above 250 mbars and Eh returned to values > 650 mV. Nitrate in extracts of soil cores sampled concurrently with measurements of gas evolution appeared to have little effect on the magnitude of the N flux. Fluxes of at least 1.20 kg N ha^–1 day^–1 were observed for nitrate levels as different as 2 and 33 µg N/g. The major impact of increased soil nitrate concentrations was in shifting the distribution of denitrification products in favor of N₂O. Annual dentrification N loss from each study site, estimated from direct measurements, compared favorably with the indirect estimate based on the imbalance in the N budget. Direct estimates ranged from 95 to 233 kg N ha^–1 year^–1 (14 to 52% of N input), and coupled with other directly measured inputs and outputs of N provided an 80 to 95% accountability of the N applied. Nitrous oxide emission comprised 13 to 20% of the total denitrification loss at sites with pH above 7, but values of approximately 30% were observed at more acid sites.

¹ Contribution from Dep. of Soil & Environmental Sciences, Univ. of California, Riverside, CA 92521. Research supported by RANN Division of National Science Foundation. Project AEN 74-11136A01.

² Research Soil Scientist and Associate Professor, respectively.

Received for publication April 4, 1979. Accepted for publication January 8, 1980.

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